Responding to Johnson's withdrawal, Boris backer and Tory MP Steve Baker told City A.M.: “It was a breathtaking announcement. I'm very disappointed, but I very much hope that his great talents and energy will be brought to bear in government and the coming election campaign."

Baker declined to comment on which of the remaining candidates would secure his support.

Chris Philp, who is backing Teresa May, said the announcement was "completely unexpected", adding that it underlined the home secretary's claims as the "stability candidate".

Pressed on why justice secretary Michael Gove could not fill the same role, Philp said: "Well, he’s gone from supporting Boris Johnson to running against him in 24 hours. People can draw their own conclusions from that."

A raft of MPs who have previously backed Johnson have also waded into the debate on social media.

Terribly sad. In doing this #Boris confounds his critics and shows why he's the best Prime Minister we'll never have. Now he's Kingmaker